INACTIVATION OF THE MITOGEN-ACTIVATED PROTEIN-KINASE MPS1 FROM THE RICE BLAST FUNGUS PREVENTS PENETRATION OF HOST-CELLS BUT ALLOWS ACTIVATION OF PLANT DEFENSE RESPONSES

The rice blast fungus, Magnaporthe grisea, generates enormous turgor p ressure within a specialized cell called the appressorium to breach th e surface of host plant cells. Here, we show that a mitogen-activated protein kinase, Mps1, is essential for appressorium penetration. Mps1 is 85% similar to yeast Slt2 mitogen-activated protein kinase and can rescue the thermosensitive growth of slt2 null mutants. The mps1-1 Del ta mutants of M. grisea have some phenotypes in common with slt2 mutan ts of yeast, including sensitivity to cell-wall-digesting enzymes, but display additional phenotypes, including reduced sporulation and fert ility, Interestingly, mps1-1 Delta mutants are completely nonpathogeni c because of the inability of appressoria to penetrate plant cell surf aces, suggesting that penetration requires remodeling of the appressor ium wall through an Mps1-dependent signaling pathway. Although mps1-1 Delta mutants are unable to cause disease, they are able to trigger ea rly plant cell defense responses, including the accumulation of autofl uorescent compounds and the rearrangement of the actin cytoskeleton. W e conclude that MPS1 is essential for pathogen penetration; however, p enetration is not required for induction of some plant defense respons es.